1. Field of the Invention
The present invention relates to a thermal-type infrared sensor including a thermopile, and an image forming apparatus including the thermal-type infrared sensor.
2. Description of the Related Art
Recently, development of a non-cooling thermal-type infrared array sensor, a thermal-type infrared line sensor, and the like using a bolometer, a thermopile, a diode, or the like has been performed actively. Since having sensitivity in a wavelength band from mid-infrared to far-infrared, these sensors are widely used, for example, in a night-vision camera for an automobile, a human body detection sensor for a security instrument, and a human body detection sensor for power-saving of an electric/electronic instrument.
Such a thermal-type infrared sensor mainly includes three components which are an infrared absorption layer, a heat insulation structure, and a temperature sensor. Generally, the infrared absorption layer and the temperature sensor are arranged on the heat insulation structure.
The heat insulation structure is generally a bridge structure, a diaphragm structure, or the like formed by a micro electro mechanical systems (MEMS) process or the like. By the heat insulation structure, sensitivity enough for weak infrared can be acquired.
Also, such a thermal-type infrared sensor and a lens are combined and used to adjust a field angle or a detection range and are applied to various applications.
Recently, in a detection method of the thermal-type infrared sensor, development of what uses a thermopile as a temperature sensor has been particularly performed. A thermopile thermal-type infrared sensor is disclosed, for example, in Japanese Laid-open Patent Publication No. 2000-65638. In Japanese Laid-open Patent Publication No. 2000-65638, to reduce bending and torsional stress on a beam, a thermopile thermal-type infrared sensor, in which an n-type polysilicon material and a p-type polysilicon material on the beam are arranged in a linearly symmetrical manner to a center line in a longitudinal direction of the beam, is disclosed.
FIG. 17 and FIG. 18 are general views for describing a conventional thermal-type infrared sensor. FIG. 17 is a plane view. FIG. 18 is an X-X′ sectional view of FIG. 17.
On a substrate portion 101, a first thermopile material 103 and a second thermopile material 104 which configure a thermopile 102 are connected by a wiring material 105 including a conductive material, whereby a thermocouple is formed. The thermocouples are connected in series in a plurality of stages by the wiring material 105, whereby the thermopile 102 is formed. The first thermopile material 103 and the second thermopile material 104 are connected by the wiring material 105 through a contact hole 106.
A hollow portion 107 formed on the substrate portion 101 forms a thin film portion 108. In a thin film section around the thin film portion 108, a plurality of through-holes 109 is formed, whereby a beam portion 110 is formed. The first thermopile material 103 and the second thermopile material 104 are arranged over the thin film portion 108 and the substrate portion 101 across the beam portion 110.
The first thermopile material 103 and the second thermopile material 104 are formed on an insulation film 111 formed on the substrate portion 101. The first thermopile material 103, the second thermopile material 104, and the wiring material 105 are covered by an interlayer insulation film 112. On the interlayer insulation film 112, an infrared absorption film 113 is formed.
In the thermopile thermal-type infrared sensor illustrated in FIG. 17 and FIG. 18, an electric conduction path of the thermopile 102 is arranged in a substantially annular shape in such a manner to surround a center section of the thin film portion 108. When a uniform magnetic field 114 passes in a direction vertical to a plane through the substantially annular conduction path formed by the thermopile 102, an induced current in a direction of an arrow 115 in FIG. 17 is generated in the thermopile 102 according to the corkscrew rule. Thus, there has been a problem that the thermopile thermal-type infrared sensor is easily influenced by an extraneous electromagnetic noise.
Therefore, there is a need for improving a resistance characteristic of a thermopile thermal-type infrared sensor to an extraneous electromagnetic noise.